I've made my fair share of slings on the cheap with materials available at my local Ace Hardware as have many others. That might be a decent place to start looking for materials.
^^^ Just saw your post after a quick edit on my last comment. Yup, I'll check ACE, too. Good store for that sort of thing.
"Figure Out What You Believe and STAND YOUR GROUND!"
Nemo Me Impune Lacessit
Lots of Cast Iron to Cook With...
and a few defensive firearms, too!